Zika infection in pregnant women has been shown to be a cause of the birth defect microcephaly and other serious brain abnormalities in babies.

The agency, which is giving public health advice to Brazil, said pregnant women should stay away from areas with ongoing Zika virus transmissions.

"This includes Rio de Janeiro. Pregnant women’s sex partners returning from areas with circulating virus should be counselled to practise safer sex or abstain throughout the pregnancy," WHO said.

Brazil is one of almost 60 countries and territories which to-date report continuing transmission of Zika by mosquitoes.

Health experts said that the risk of catching the virus in Rio de Janeiro during the Olympics was lower than in many other parts of the Americas.

Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine, agreed with the WHO that there was no public health reason to cancel or delay this summer's Games.

"The Olympics will be held in August, the coldest month of the year in Rio de Janeiro, when the mosquito numbers will be at their lowest," Hotez said.

"This virus really raged through a year ago and a significant percentage of the population has already been immunised, so that will further reduce transmission. And the Brazilians have been preparing for the Olympics by aggressively applying insecticides to the area around Rio."

Hotez acknowledged there was some risk but he maintained it was lower than elsewhere.

"It will be far lower than in many other parts of the Western hemisphere, certainly compared to Central America or the Caribbean or even the US Gulf coast. Rio de Janeiro may even be one of the safer places," he said.

"For all these reasons i do not see a compelling reason to cancel the Games at this point."

The tropical disease expert was backed up by Tom Frieden, director of the US Centers for Disease Control and Prevention, who also said there was no health reason to cancel or delay the Games.

'Infection risk too high'

Their assessment runs counter to the public letter posted online by the group of leading public health experts, many of them bioethicists, who said the risk of infection from the virus was too high.

The letter was sent to Dr Margaret Chan, director-general of the World Health Organization, and urged that the Games be moved to another location or delayed.

"An unnecessary risk is posed when 500,000 foreign tourists from all countries attend the Games, potentially acquire that strain, and return home to places where it can become endemic," the letter said.

Professor Arthur Caplan, director of the division of medical ethics at New York University School of Medicine and one of four who authored the letter, said he was sceptical Brazil had the resources to protect the public and was equally skeptical of "general assurances" from public health officials.

The letter called on the WHO to convene an independent group to advise it and the International Olympic Committee.

"I believe in informed consent," Caplan said in an interview. "Let's have an independent set of scientists look at this and let everyone hear the arguments."